How can output of "copy rotation constraint" be accessed?
Use the world matrix.

On Q2 target_object.matrix_world.to_euler() will give you the resulting Euler rot, which could be used in a panel to ans Q1 part.

Example script using the panel template. 

import bpy
from math import degrees

class LayoutDemoPanel(bpy.types.Panel):
    """Creates a Panel in the object context of the properties editor"""
    bl_label = "Layout Demo"
    bl_idname = "OBJECT_PT_layout"
    bl_space_type = 'PROPERTIES'
    bl_region_type = 'WINDOW'
    bl_context = "object"

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def draw(self, context):
        layout = self.layout
        obj = context.active_object
        col = layout.column()
        mw = obj.matrix_world
        euler_rot = mw.to_euler()
        for axis in "xyz":
            col.label(text="%c : %7.4f" % (axis, degrees(getattr(euler_rot, axis, 0))))

def register():
    bpy.utils.register_class(LayoutDemoPanel)

def unregister():
    bpy.utils.unregister_class(LayoutDemoPanel)

if __name__ == "__main__":
    register()